How do you know if crank shaft is broke?

A broken crankshaft on a Continental Gasoline fueled truck engine from the 1940s and 50s would begin knocking like a failed connecting rod and continue to run for about 20 to 40 minutes. (great for a fire truck)

A Chrysler built 2.2 the easy diag is that the timing belt is intact but does not move when rolling the engine over on the starter.

Most engines will have little or no compression and resistance to the starter turning the engine, on closer examination it will be found the camshafts are not turning. After removing the oil pan it will be obvious.